media: imx-pxp: extract helper function to setup data path

The driver must configure the data path through the Pixel Pipeline.

Currently, the driver is using a fixed setup, but once there are
different pipeline configurations, it is helpful to have a dedicated
function for determining the register value for the data path.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>